The present invention relates to a process for producing lead wires of hard carbon for supplying power to a fuel battery. More particularly, the invention relates to an inexpensive and simple industrial process for producing lead wires of hard carbon having excellent corrosion resistance against electrolyte in a fuel battery, light weight, high hardness and high mechanical strength.
Lead wires for supplying power to a fuel battery are generally desired to satisfy the following conditions:
(1) Electricity generated with little electric resistance and which is not consumed as Joule heat. PA0 (2) Excellent corrosion resistance against electrolyte of high temperature and high acid, alkaline or solubilized salt solution concentrations, and without deterioration in quality over an extended period. PA0 (3) Light weight so that the weight of the entire fuel battery device is not excessively large. PA0 (4) High surface hardness and excellent wear resistance. PA0 (5) High mechanical strength, such as bending strength and Young's modulus. PA0 (6) Industrially simple and inexpensive to produce.
Heretofore, lead wires used for general power purposes have been made of metal wires such as copper wires. However, the metal wires are not appropriate as lead wires of a fuel battery in regard to durability against high temperature electrolyte. Novel metals, such as gold and platinum are better than copper against corrosion resistance, but still are not satisfactory, and are excessively expensive and accordingly are not suitable for use. Metals generally have a large specific weight which is considered to be disadvantageous because the weight of the fuel battery device is excessively increased.
On the other hand, carbon materials have been used as electrodes in the electrolytic and dry battery industries, etc., due to excellent features such as excellent electric conductivity, corrosion resistance, and light weight. A process for producing the electrodes includes the steps of using coke powder and graphite powder as fillers, and pitches as binder such as petroleum pitch and coal pitch, mixing and kneading both, extrusion molding the mixture, injection molding the mixture, calcining to carbonize the mixture, and graphiting the carbonized mixture as required. However, when considering the thickness of the unit cell of a fuel battery and the specific resistance of the carbon material, lead wires for supplying power in the fuel battery preferably employ carbon rods having a diameter of several mm or shorter, and more preferably 2 to 4 mm. However, since the mechanical strength of the carbonaceous material according to the above process is extremely small, such as 50 MPa at the maximum, the carbon rods having a thickness of 2 to 4 mm lack absolute strength, and are not proper for use.
Recently, as the carbon material industry was developed, self-sintered carbon material was obtained by using mesocarbon microbeads, semicoke powder or coking coal powder, molding the material by an isostatic pressing method, etc., carbonizing the material and then graphiting the carbonized material. This process has been used for artifical graphite electrodes having high strength. However, since these materials are obtained in a block state due to the restriction in the producing process, it is necessary to cut the material by machining for use as lead wires. However, the mechanical strength of the obtained product is lower than the base material because of damage or cracks produced on the carbon material from the mechanical workings. In addition, even if a high strength product is obtained having a maximum mechanical strength of 1000 MPa of bending strength, such strength is not sufficient. The materials are economically expensive considering the long steps of cutting and damage during working due to insufficient mechanical strength.
The artifical graphite materials have further drawbacks in relation to the electrolyte. When artificial graphite materials are contacted with a high concentration of acid or solubilized salt, the material between graphite layers is easily deteriorated in its quality. Further, surface hardness is small and the materials are easily worn. Accordingly, such materials are not satisfactory as lead wires for a fuel battery.
The inventor has previously invented a process for producing vitreous carbon fine wires made by mixing and dispersing a mixture composition containing chlorinated vinyl chloride resin as the main ingredient, extrusion molding the kneaded material in a fine wire state, carbon pretreating the resultant molding, and then calcining the carbon pretreated molding in an inert gas atmosphere. This process was filed as a patent application under Japanese Patent Laid-open No. 86410/1986. Since the carbon obtained by this process is vitreous, it has the advantages of excellent corrosion resistance against electrolyte, high surface hardness, high mechanical strength, and light weight. However, the carbon has a drawback in that its specific resistance is relatively large, i.e., on the order of 10.sup.-3 ohm-cm. In order to avoid this drawback, the diameter of the lead wire made of carbon must be large. Further, according to this process, since the main ingredient of the mixture composition is solely the chlorinated vinyl chloride resin, it has such a drawback that the carbon pretreating step of molding is slow, and it is extremely difficult to form a diameter of 1 mm or more of the obtained product after carbonization, and it is not preferable as an effective lead wire.